The present invention relates to a blanking cutter for punching out a dumbbell-shaped test piece from a sheet of rubber, plastic, vinyl, polyethylene, paper, or the like.
Various materials such as rubber, plastic, vinyl, polyethylene, paper, etc. are tested for mechanical strength using a dumbbell-shaped test piece of such a material, as shown in FIG. 7 of the accompanying drawings. The dumbbell-shaped test piece has widened opposite ends 2 each having a width B1 and an intermediate narrower strip portion 3 having a width B2. The opposite ends 2 have lateral sides smoothly joined along sine curves to the lateral sides of the intermediate strip portion 3 via convex curves 4a having a radius of curvature r1 and concave curves 4bhaving a radius of curvature r2. The shape and dimensions of such a dumbbell-shaped test piece are specified in detail according to various industrial standards including JIS (Japan), DIN (Germany), and ASTM (U.S.A.).
Heretofore, dumbbell-shaped test pieces are cut out of a sheet by a forged blanking cutter 5 as shown in FIG. 11. The blanking cutter 5 has an upper edge 6 forged to a shape complementary to the shape of a dumbbell-shaped test piece to be blanked. The upper edge 6 is sharpened by a grinder or the like into a shearing blade, which is subsequently heat-treated for desired hardness. The shearing blade is however relatively low in dimensional accuracy because it is sharpened manually by the grinder. If the shearing blade is broken or damaged, it is necessary to replace the entire blanking cutter 5, which is not economical.
In view of the above drawback, the applicant has proposed a blanking Cutter 100 with replaceable shearing blades, as shown in FIGS. 8 through 10 (see Japanese Laid-Open Patent Publication No. 54(1949)-60389).
The blanking cutter 100 comprises a base 101 having a shape complementary to the shape of a dumbbell-shaped test piece to be blanked, four presser plates 102a, 102b, 103a, 103b fastened intimately to the respective four sides of the base 101, four shearing blades 105a, 105b, 106a, 106b clamped between the base 101 and the respective presser plates 102a, 102b, 103a, 103b, and a grip boss 107 joined to an upper surface of the base 101. Each of the shearing blades 105a, 105b, 106a, 106b is in the form of a thin plate having a thickness of 0.3 mm, for example, and is so flexible that the shearing blades 105a, 105b can intimately be held against the curved lateral sides of the base 101 by the presser plates 102a, 102b. The shearing blades 105a, 105b, 106a, 106b have lengths equal to the extended lengths of the corresponding four sides of the base 101. The shearing blades 105a, 105b, 106a, 106b pressed by the respective presser plates 102a, 102b, 106a, 106b and fastened to the base 1 have respective cutting edges projecting downwardly of the base 101 and the presser plates 102a, 102b, 103a, 103b.
The blanking cutter 100 is attached to a press by the grip boss 107. Then, the blanking cutter 100 is held against a sheet, and forced into the sheet by the press, thereby cutting out a dumbbell-shaped test piece from the sheet.
It has been found that the cutting edges of the shearing blades 105a, 105b for defining the curved sides of a dumbbell-shaped test piece tend to be deflected inwardly as indicated by the two-dot-and-dash lines A in FIG. 9. The deflected shearing blades 105a, 105b are shown exaggerated for illustrative purpose, and the actual amount of deflection of the shearing blades 105a, 105b is smaller. Since dumbbell-shaped test pieces are required to have high dimensional accuracy, if the shearing blades 105a, 105b are deflected as shown in FIG. 9, it is difficult to produce dumbbell-shaped test pieces which meet desired accuracy requirements. If a dumbbell-shaped test piece 1 (FIG. 1) were produced by the blanking cutter 100 with deflected shearing blades, then the width B2 of the intermediate strip portion 3 and the shape of the curves 4a, 4b would be subjected to error. Any material test using such a dumbbell-shaped test piece 1 with dimensional errors would be unable to achieve proper results.